We have developed a clinically useful method (the laser Doppler instrument and its theory of operation) for clinical measurements of microcirculatory blood flow, the density of flowing red blood cells (RBCs), and mean RBC velocity. Assistance has been given to the commercialization of this methodology and to its application to clinical research. Our clinical studies have been directed toward examining normal and abnormal microvascular dynamics through noninvasive clinical studies of skin and nasal mucosa, and toward intraoperative clinical studies of muscle and of the CNS. Considerable theoretical work has been directed toward finding an adequate construct with which to interpret the physical measurements and refining the accuracy of the microcirculatory measurements. We have discovered abnormal microcirculatory patterns and responses in the skin of patients with sickle-cell disease, hypertension, certain cardiac circulatory syndromes, diabetes, and skin cancer. The microcirculatory effects of therapy are monitored with this technique, affording a better understanding of the microcirculatory components of these diseases. We are now engaged in a clinical protocol to determine if LDF measurements showing increases in skin blood flow as a function of cumulative radiation dose to the skin during radiation therapy of a variety of deeper tumors may be predictive of the general tumor and tissue radiation sensitivity of a given patient.